Field of the Invention
The invention relates to a method and device for controlling and regulating a dryer located downstream of a varnishing unit in a rotary printing press.
Rotary printing presses with varnishing devices and dryers located downstream therefrom for drying printing stock varnished in the varnishing devices have become known heretofore in the prior art. For example, sheets which have been printed in printing units of sheet-fed rotary offset printing presses are coated with a varnish layer in a varnishing unit disposed downstream of the printing units, and the sheets are then guided past a conventional dryer in the form of an infra-red (IR), hot-air, or ultra-violet (UV) dryer, wherein the varnished sheets are dried before being deposited on a delivery pile.
The pressman operating heretofore known sheet-fed rotary offset printing presses is faced with the problem that the sheets will stick together or adhere when deposited on a sheet pile in the delivery system, if the varnish remains damp or is too damp, in the case where the dryer power is too low and the sheets remain in the dryer for too short a time, respectively. To counteract adherence or sticking of the sheets, they are therefore coated in the delivery region with a layer of powder in a conventional manner, which not only increases operating costs but also leads to unnecessary soiling and to an impairment of print quality.
On the other hand, if the dryer power is set too high or the sheets remain too long in the dryer, the viscosity of the varnish that has already begun to dry is reduced again because of the higher temperature, so that, when the sheets are deposited on the sheet pile, they become stuck together, in like manner. In addition, the energy consumption of the dryer, which is already a major energy consumer, is further increased. To enable the dryers to be set or adjusted in practice to a more or less optimal operating range, it has become known heretofore from the Speedmaster 102 series of Heidelberger Druckmaschinen A. G., the corporate assignee of the instant application, to monitor the temperature of the delivery pile with a temperature sensor, so that if a given value for the sheet pile temperature is exceeded, the pressman can lower the power of the dryer. Then, however, the problem arises that the measured pile temperature does not provide the dryer with any indication of a lower limit for the dryer power that is outputted. In order to obtain such a lower limit for the dryer power at the application production run speed, a subjective assessment of the degree or extent of drying of the varnished sheets laid on the sheet pile is made by the pressman, in practice, through the use of his fingers, for example, by checking the tackiness of the varnish. Depending upon what he or she finds, the pressman or other operator then selects the dryer power and the quantity of powder used for a given production run speed and pile height, so as to just barely prevent the sheets from sticking together. Because of this subjective measuring method to set or adjust the optimal dryer power, in practice, disruptions in operation often occur, along with unnecessarily high powder consumption and high incidence of spoiled copies.